The present invention relates to a process for preparing diacetoxybutene. Diacetoxybutene is an important raw material for the production of 1,4-butanediol and tetrahydrofuran which are utilized as a solvent or synthetic resin material.
Diacetoxybutene is usually produced by reacting butadiene, acetic acid and oxygen in the presence of a palladium catalyst. It is necessary to separate diacetoxybutene from the acetoxylation reaction product, but some problems are involved in this process. Since both of the unreacted materials and diacetoxybutene contained in the reaction product have unsaturated groups (unsaturated radicals), there can be generated polymeric substances as a by-product depending on the treatment involved, and such generation of the polymeric substances as a by-product may become a cause of decrease of yield or operational impediment.
Japanese Patent Application Laid-Open (KOKAI) No. 50121210 (corresponding to U.S. Pat. No. 4,057,472) teaches that diacetoxybutene can exist in relatively stable when distillation for separating diacetoxybutene from the said reaction product is carried out by controlling the pressure within the distillation tower while maintaining the temperature of the bottom of the distillation tower at lower than 190.degree. C., and as a result, the above-mentioned problems have been solved.
However, for separating and recovering diacetoxybutene from the reaction product in a high yield, it is essentially required to prolong the residence time of the reaction products in the distillation tower. Therefore, even if the temperature of the bottom of the distillation tower is kept low, for example, not higher than 190.degree. C., it is hardly possible to perfectly prevent occurrence of polymerization and decomposition, and there can arise the problems such as mixing of the decomposed substances in the distillate diacetoxybutene from the distillation tower and blockage of an outlet for withdrawing the bottoms due to deposition of the polymeric substances to the heater at the bottom of the distillation tower. Thus, in the conventional processes, in order to avoid the abovementioned problems it has been inevitable that bottoms of the distillation tower containing diacetoxybutenes as well as high boiling point materials be discarded.
As a result of the present inventors' strenuous studies for satisfying the demands, it has been found that in the process for preparing diacetoxybutene comprising reacting butadiene, acetic acid and oxygen in the presence of a palladium-based catalyst, distilling the reaction product in a first distillation tower to distill off acetic acid and water, and further distilling the residual materials in a second distillation tower, by withdrawing the obtained bottoms from the second distillation tower in a situation where the content of high boiling point materials in the bottoms of the second distillation tower is not raised above 20%, subjecting the withdrawn bottoms to evaporation treatment in a thin-film evaporator, and returning the evaporated materials containing diacetoxybutene to the second distillation tower, it is possible to obtain diacetoxybutene with higher purity in a higher yield. The present invention has been attained on the basis of this finding.